A main rotor pylon in a helicopter aircraft provides several key features. First, the main rotor pylon shrouds the helicopter transmission, the engines, the engine exhausts., and the equipment bays preventing contaminants from entering into them. Second, the pylon also provides a walking surface for maintenance personnel to access and inspect the main rotor system. The helicopter main rotor requires considerable maintenance. Hence, the ability to use the main rotor pylon as a walking surface is especially important.
Conventional main rotor pylons include an external skin affixed to an internal support structure. The support structure extends from just forward of the engine compartment past the engine exhaust. A conventional main rotor pylon support structure is shown in FIG. 1 and typically includes metallic framing members and shear panels. The shear panels function in conjunction with the framing members to provide the necessary support to accommodate the anticipated walking loads which is prescribed by government standard as 450 pounds. The framing members include upper caps bolted or riveted to spaced vertical stiffeners. The vertical stiffeners, in turn, are bolted or riveted to the top of the helicopter cabin. The caps and vertical stiffeners are typically either channel, T or L-shaped components. The sheet metal shear panels are riveted to the caps, the vertical stiffeners and the cabin to form a rigid support structure.
One FAA requirement imposed on many larger helicopters is that a fireproof wall must separate any compartments located adjacent to an engine or exhaust compartment. Since the main rotor pylon support structure is located adjacent to the engine compartment and the engine exhaust compartment, the materials used to fabricate the structure must be selected so as to prevent fire from passing from these compartments into the interior of the main rotor pylon. Also, the support structure must be designed to accommodate the normal heat that is generated around the engine or exhaust, which can be in excess of 1000.degree. F. These requirements have, up to the present day, necessitated the use of titanium and/or steel for the framing members, the shear panels and cabin skins.
There are several deficiencies with the prior art main rotor pylons. First, titanium and steel are relatively expensive materials and are heavier than aluminum and composite structures. The additional weight typically requires the incorporation of additional stiffening members. Furthermore, the manufacturing of conventional support structures has required that the framing structures, i.e., the frame caps and the vertical stiffeners, be painstakingly riveted to the shear panels in order to form the support structure. Hence, the time to fabricate a conventional support structure has been quite considerable.
Furthermore, since the framing members and shear panels are all riveted to one another, it is important to accurately fabricate and attach these components in order to prevent the need for modifications during the assembly process. The difficulty in maintaining this accuracy is evidenced by the fact that if one were to place two actual support structures side be side, there would be slight differences in the structure. While these differences are not critical to the structural integrity of the components, any modifications that are required typically add additional time to the manufacturing process to ensure the proper fit or interchangeability of those components.
In the conventional support structure, since the shear panels are riveted to the framing members and the cabin, it is not possible to readily access the engine and/or internal compartments through the shear panels. As such, doors must be added to the panels if access is needed, increases the time and cost associated with fabricating the support structure.
Another deficiency that results in conventional shear panel structures is the development of cracks. Shear panels tend to crack when subjected to alternating tension and compression loads. These alternating loads tend to cause the shear panel to "oil can." Oil canning is the phenomenon where the panel deforms out of plane. The alternating loads can be the result of takeoff and landing which induce somewhat sudden loads on the aircraft support structure. When a crack develops in a shear panel, the panel must be replaced or repaired.
Conventional main rotor pylon support structures are also designed to transfer some of the applied loads directly to the cabin skin, instead of to an attachment point, such as a helicopter frame. This is an inefficient way of transferring the walking loads into the aircraft and may necessitate the addition of further stiffening elements to provide adequate structural support for the cabin skin.
A need, therefore, exists for an improved main rotor pylon support structure which provides fire protection, is easy to manufacture, provides increased aircraft accessibility, and is light in weight.